The increase in body fat and the loss of fat-free mass that occur during aging in adult humans are thought to be undesirable because they are associated with a number of health risks and changes in body function. The metabolic causes of these changes in body composition, and the extent to which they are influenced by dietary practise and lifestyles, are not understood fully, and will be the focus of the proposed project. New, non-invasive methods will be used to test the hypothesis that aging is associated with alterations in the metabolic and appetite responses to varying levels of dietary intake, and that these alterations contribute to an increase in body energy deposition during postitive energy balance, and a reduced mobilization of energy and an increased loss of fat-free mass during negative energy balance. The approach to be taken is as follows: Total energy expenditure, components of expenditure, energy and protein balance, will be measured during 20 days of approximate energy balance and during 20 days of moderate overfeeding or underfeeding, in healthy adults aged 20-25, 70-75 years. The role of an increase in the proportion of dietary energy derived from protein in changes in energy utilization associated with aging will also be addressed, by using two diets in the overfeeding study (with adequate or generous protein contents). Detailed metabolic studies of energy substrate cycling and protein turnover will be conducted, to examine the biochemical basis of changes in energy expenditure related to aging, overfeeding and underfeeding. Measurements of voluntary food intake after the overfeeding or underfeeding will be used to assess age-related alterations in the control of food intake. From these data, it will be possible to describe in detail the effect of aging in humans on the metabolic adaptations and responses to variations in dietary intake.